My baby is now 10 months old. He cannot say "papa" or "mama". He can only pronounce some meaningless tones. I have heard that baby boys are slower. Some will only address their parents at about age 1.5 or even 2.
When my baby was 8 months old, he could not sit without aid. I went to see the doctor. She said we need to train him. So we trained him by using something he likes (a soybean milk box in our case, you can use your baby's favourite toy). We held the box up so he started to grasp the box and balance his body. After half an hour, he could sit. We then practise this 3 times a day with half an hour each time. After one week, he could sit properly without any aid. You may wish to try this.